The field of switching power converters is a rich one, with many classes of inventions spanning several decades. The present disclosure belongs to the class of AC-DC converters in which AC power is input from supply mains, converted to high voltage DC, then switched through a transformer and rectified to supply regulated DC power which is electrically isolated from the mains for safety. Switching frequencies are much higher than the mains frequencies, ranging from tens of kilohertz to several megahertz.
Regulating the DC output voltage against load and mains variations requires a feedback control system to sample the output and to control the applied power in an appropriate manner to counter such variations in cases where the output is electrically isolated from the mains, providing feedback for the control system is complicated by the requirement of maintaining very high impedance (ideally, infinite) between input and output sections of the converter. Because of this isolation, sometimes regulator circuits are designed which sample and regulate the transformer input. But this puts the transformer and following rectifier outside the control loop, raising the output impedance of the converter and degrading the regulation. So, for sampling the output itself, it is common practice to communicate the state of the output voltage by transformer or optical coupling to the input section. Recent practice has favored the use of optical signal-couplers (usually called "optocoupiers"), which are small and inexpensive and which have a frequency response extending to DC.
The most common technique for varying the applied power is to control the duty cycle of the switching waveform (ie, pulsewidth modulation). When the output voltage drops, the control system adjusts the pulsewidth to supply more power to the load, and conversely for a rising output voltage. Such a control system is a sampled-data system operating in a linear mode. The sampling rate is the frequency of the switching waveform. Putting an optocoupler in the feedback complicates the loop design because optocouplers exhibit rather large variations of transfer function gain from unit to unit. (Optocouplers consist of a combination of a light emitting diode and a phototransistor, often with wide production variations in the characteristics of each element). The gain variation can be as high as 4:1 in a batch of modest cost units, and this fact usually requires designing the control loop to have less than optimum gain in order to insure stability. Lower loop gain implies, of course, more output voltage variation and slower transient response. The alternatives are either tight specifications on the optocoupler or a variable gain control, either of which defeats the important goal of achieving low cost in mass-produced products.
Clearly, what is needed for isolated converters is a way to use inexpensive optocouplers in a "tight" and fast controller circuit.